Multi-step hydrodesulphurisation process

ABSTRACT

A hydrodesulfurization process is provided for continuously effecting hydrodesulfurization of a liquid sulfur-containing hydrocarbon feedstock which comprises: (a) providing a hydrodesulfurization zone maintained under hydrodesulfurization conditions and comprising a column reactor having a plurality of reaction trays therein mounted one above another, each tray defining a respective reaction stage adapted to hold a predetermined liquid volume and a charge of a sulfided solid hydrodesulfurization catalyst therein, liquid downcomer means associated with each reaction tray adapted to allow liquid to pass down the column reactor from that tray but to retain solid catalyst thereon, and gas upcomer means associated with each reaction tray adapted to allow gas to enter that tray from below and to agitate the mixture of liquid and catalyst on that tray; (b) supplying liquid sulfur-containing hydrocarbon feedstock to the uppermost one of said plurality of reaction trays; (c) supplying hydrogen-containing gas below the lowermost one of said plurality of reaction trays; (d) allowing liquid to pass downward through the column reactor from tray to tray; (e) allowing hydrogen-containing gas to pass upward through the column reactor from tray to tray; (f) recovering from the uppermost one of said plurality of reaction trays and off-gas containing hydrogen sulfide produced by hydrodesulfurization; and (g) recovering from the lowermost one of said plurality of reaction trays a liquid hydrocarbon product of reduced sulfur content.

FIELD OF THE INVENTION

This invention relates to a process for hydrodesulphurisation of ahydrocarbon feedstock.

DESCRIPTION OF BACKGROUND ART

Crude oils, their straight-run and cracked fractions and other petroleumproducts contain sulphur in varying amounts, depending upon the sourceof the crude oil and any subsequent treatment that it may haveundergone. Besides elemental sulphur, numerous sulphur compounds havebeen identified in crude oil including hydrogen sulphide (H₂ S), C₁ toC₅ primary alkyl mercaptans, C₃ to C₈ secondary alkyl mercaptans, C₄ toC₆ tertiary alkyl mercaptans, cyclic mercaptans (such as cyclopentanethiol, cyclohexane thiol and cis-2-methylcyclopentane thiol), open chainsulphides of the formula R-S-R' where R and R' represent C₁ to C₄ alkylgroups, mono-, bi- and tri-cyclic sulphides, thiophene, alkylsubstituted thiophenes, condensed thiophenes (such as benzo(b)thiophene,isothionaphthene, dibenzothiophene, andbenzo(b)naphtho(2,1-d)thiophene), thienothiophenes, alkyl cycloalkylsulphides, alkyl aryl sulphides, 1-thiaindans, aromatic thiols (such asthiophenol), and cyclic thiols such as cyclohexane thiol.

SUMMARY OF THE INVENTION

Generally speaking, low API gravity crude oils usually contain moresulphur than high API gravity crude oils, although there are someexceptions. Moreover the distribution of sulphur compounds in thedifferent fractions of petroleum varies mainly with the boiling range ofthe fractions. Thus the lighter fractions such as naphtha contain fewersulphur compounds, whilst the content of sulphur compounds alsoincreases as the boiling point or API density or molecular weight of thefraction increases. Most of the sulphur compounds that have beenpositively identified as components of crude oil boil below about 200°C. Many other sulphur compounds of high molecular weight and highboiling point remain unidentified in crude oil.

For a variety of reasons it is necessary to treat crude oil andpetroleum fractions derived therefrom to remove the sulphur componentspresent therein. Otherwise subsequent processing may be hindered, forexample because the sulphur components may adversely affect theperformance of a catalyst. If the hydrocarbon fraction is intended forfuel use, then burning of the fuel will result in any sulphur componentspresent therein being converted to sulphur oxides which areenvironmentally damaging.

For these reasons it is necessary to remove as far as possible thesulphur content from hydrocarbon fractions derived from crude oil, suchas gasoline fractions, diesel fuel, gas oils and the like. Typicallysuch sulphur removal is carried out by a process known generally ashydrodesulphurisation. In such a process the hydrocarbon fraction isadmixed with hydrogen and passed over a hydrodesulphurisation catalystunder appropriate temperature and pressure conditions. In such a processthe aim is to rupture the carbon-sulphur bonds present in the feedstockand to saturate with hydrogen the resulting free valencies or olefinicdouble bonds formed in such a cleavage step. In this process the aim isto convert as much as possible of the organic sulphur content tohydrocarbons and to H₂ S. Typical equations for major types of sulphurcompounds to be hydrodesulphurised are shown below: ##STR1##

Generally the cyclic sulphur-containing compounds are harder tohydrogenate than the open chain compounds and, within the class ofcyclic sulphur-containing compounds, the greater the number of ringsthat are present the greater is the difficulty in cleaving thecarbon-sulphur bonds.

Besides the presence of sulphur oxides in the combustion gases fromhydrocarbon fuels, other environmentally undesirable components of suchcombustion gases typically include aromatic hydrocarbons, which may bepresent because of incomplete combustion, and carbonaceous particulatematter often containing polycyclic aromatic hydrocarbons, metalcompounds, oxygenated organic materials, and other potentially toxicmaterials.

Because of present concerns about pollution, increasingly stringentlimits are being placed by various national legislations around theworld upon the levels of permitted impurities in hydrocarbon fuels, suchas diesel fuel. In particular the United States Environmental ProtectionAgency has recently proposed rules which would limit the sulphur contentto 0.05 wt % and the aromatics content to 20 volume % in highway dieselfuels (see, for example, the article "Higher Diesel Quality WouldConstrict Refining" by George H. Unzelman, Oil and Gas Journal, Jun. 19,1987, pages 55 to 59). Such rules require refiners to face additionaldiesel treating requirements and increased investment and operatingcosts. Additional reductions in the permitted levels of sulphur contentand aromatics content at some future date cannot be ruled out.

When a hydrocarbon feedstock is treated with hydrogen in the presence ofa suitable catalyst with the aim of effecting hydrodesulphurisation,other reactions may also occur. Hence hydrotreating is often used as amore general term to embrace not only the hydrodesulphurisationreactions but also the other reactions that occur, includinghydrocracking, hydrogenation and other hydrogenolysis reactions. Theterm "hydrotreating" is further explained in an article "Here is anomenclature-system proposed for hydroprocessing", The Oil and GasJournal, Oct. 7, 1968, pages 174 to 175.

There are four main hydrogenolysis reactions, of whichhydrodesulphurisation (HDS) is probably the most important, followed byhydrodenitrogenation (HDN), hydrodeoxygenation (HDO), andhydrodemetallation (HDM). Amongst catalysts which have been proposed forsuch hydrotreating reactions are molybdenum disulphide, tungstensulphide, sulphided nickel-molybdate catalysts (NiMoS_(x)), andcobalt-molybdenum alumina sulphide (Co-Mo/alumina).

Although the prior art regards the simultaneous occurrence of somehydrogenation reactions, such as hydrogenation of olefins and aromatichydrocarbons, as not being advantageous in a hydrodesulphurisationprocess because the aromatic content of the product was within therequired specification and because the use of valuable hydrogen forunnecessary hydrogenation reactions was deemed disadvantageous, there isa growing storage of light crude oil. Thus the present and future trendtowards the use of middle distillates and heavier petroleum fractions,coupled with increasingly stringent specifications, means that aromatichydrogenation will be an increasingly necessary component of refineryoperations. Hence, under current conditions and increasingly for thefuture, it will be desirable to combine hydrodesulphurisation andaromatic hydrogenation.

In contrast, except when processing high molecular weight residues,extensive hydrocracking reactions are to be avoided in most refineryhydrotreating operations as far as possible because they are highlyexothermic and can cause thermal damage to catalysts and reactionvessels, as well as leading to the deposition of carbonaceous materialscausing loss of catalyst activity. Thus an operator of ahydrodesulphurisation plant has reported in an article "Refiners seekimproved hydrogen production", Oil & Gas Journal, Jul. 20, 1987, pages48 and 49, that reactors in service have overheated severely, one to thepoint of rupture, due to unwanted hydrocracking reactions occurring.

The danger of such hydrocracking reactions occurring can be minimised byensuring that the catalyst remains adequately sulphided.

A number of papers have appeared in the literature relating tohydrodesulphurisation technology, including:

(a) "Kinetics of Thiophene Hydrogenolysis on a Cobalt MolybdateCatalyst", by Charles N. Satterfield et al, AICHE Journal, Vol. 14, No.1 (January 1968), pages 159 to 164;

(b) "Hydrogenation of Aromatic Hydrocarbons Catalysed by SulfidedCoO-MoO₃ /gamma-Al₂ O₃. Reactivities and Reaction Networks" by Ajit V.Sapre et al, Ind. Eng. Chem. Process Des. Dev, Vol. 20, No. 1, 1981,pages 68 to 73;

(c) "Hydrogenation of Biphenyl Catalyzed by Sulfided CoO-MoO₃ /gamma-Al₂O₃. The Reaction Kinetics", by Ajit V. Sapre et al, Ind. Eng. Chem.Process Des. Dev, Vol. 21, No. 1, 1982, pages 86 to 94;

(d) "Hydrogenolysis and Hydrogenation of Dibenzothiophene Catalyzed bySulfided CoO-MoO₃ /gamma-Al₂ O₃ : The Reaction Kinetics" by D. H.Broderick et al, AIChE Journal, Vol. 27, No. 4, July 1981, pages 663 to672; and

(e) "Hydrogenation of Aromatic Compounds Catalyzed by Sulfided CoO-MoO₃/gamma-Al₂ O₃ " by D. H. Broderick et al, Journal of Catalysis, Vol. 73,1982, pages 45 to 49.

A review of the reactivity of hydrogen in sulphide catalysts, such asthose used as hydrotreating catalysts, appears on pages 584 to 607 ofthe book "Hydrogen Effects of Catalysis" by Richard B. Moyes, publishedby Marcel Dekker, Inc. (1988).

A review of industrially practised hydrotreating processes is publishedeach year in the Journal "Hydrocarbon Processing", normally in theSeptember issue. For example reference may be made to "HydrocarbonProcessing", September 1984, page 70 et seq and to "HydrocarbonProcessing", September 1988, pages 61 to 91.

An outline of three prior art hydrotreating processes appears in"Hydrocarbon Processing 1988 Refining Handbook" on pages 78 and 79 of"Hydrocarbon Processing", September 1988. In the "Chevron RDS/VRDSHydrotreating Process" a mixture of fresh liquid hydrocarbon feedstock,make-up hydrogen and recycled hydrogen is fed to a reactor in a"once-through" operation. As illustrated the reactor has three beds andinter-bed cooling is provided by injection of further amounts of recyclehydrogen. The recycle hydrogen is passed through an H₂ S scrubber. Inthe "HYVAHL Process" a once-through operation for the liquid feed isalso used. Again, amine scrubbing is used to remove H₂ S from therecycle hydrogen. The Unionfining Process also utilises a once-throughbasis for the liquid feed. Cocurrent hydrogen and liquid flow isenvisaged. Unreacted hydrogen is recycled.

In all three processes gas recycle is used to cool the catalyst bed andso minimise the risk of thermal runaways occurring as a result ofsignificant amounts of hydrocracking taking place. Use of gas recyclemeans that inert gases tend to accumulate in the circulating gas whichin turn means that, in order to maintain the desired hydrogen partialpressure, the overall operating pressure must be raised to accommodatethe circulating inert gases and that the size and cost of the gasrecycle compressor must be increased and increased operating costs mustbe tolerated.

Use of a trickle technique is described in an article "New ShellHydrodesulphurisation Process Shows These Features", Petroleum Refiner,Vol. 32, No. 5 (May 1953), on page 137 et seq. FIG. 1 of this articleillustrates a reactor with four catalyst beds with introduction of amixture of hot gas and gas oil at the inlet end of the first bed and useof cold shots of gas oil between subsequent beds.

In these hydrodesulphurisation processes the conditions at the inlet endof the catalyst bed are critically important because this is where therisk of hydrocracking is greatest, especially if the level ofsulphurisation of the catalyst should drop. This can occur, for example,if a low sulphur feedstock is fed to the plant or if a feedstock is usedin which the sulphurous impurities are predominantly polycycliccompounds.

Hydrorefining of a naphtha feedstock is described in U.S. Pat. No.4,243,519. This appears to involve a substantially wholly vapour phaseprocess.

Multiple stage hydrodesulphurisation of residuum with movement ofcatalyst between stages in the opposite direction to movement of gas andliquid is described in U.S. Pat. No. 3,809,644.

U.S. Pat. No. 3,847,799 describes conversion of black oil to low-sulphurfuel oil in two reactors. Make-up hydrogen is supplied to the secondreactor but in-admixture with hydrogen exiting the first reactor thathas been purified by removal of hydrogen sulphide therefrom. Hencehydrogen is recovered from the first reactor and recycled to the secondreactor in admixture with inert gases-which will accordingly tend toaccumulate in the gas recycle loop. Any condensate obtained from thefirst reactor is admixed with product from the second reactor.

In a hydrodesulphurisation plant with a gas recycle regime some of theH₂ S produced, normally a minor part thereof, will remain in the liquidphase after product separation whilst the remainder, normally a majorpart thereof, of the H₂ S will remain in the gas phase. Even in plantsin which interbed cooling with "cold shots" of recycle gas is practisedthe H₂ S released remains in the gas/liquid mixture as this passesthrough the catalyst bed. Hence the H₂ S partial pressure is-usuallyhighest at the exit end of the catalyst bed or of the final bed, if morethan one bed is used. As the catalyst activity for hydrodesulphurisationis decreased by raising the H₂ S partial pressure, the catalyst activityis lowest at the exit end from the bed-which is where the highestactivity is really needed if the least tractable polycyclic organicsulphurous compounds are to undergo hydrodesulphurisation.

The catalysts used for hydrodesulphurisation are usually also capable ofeffecting hydrogenation of aromatic compounds, provided that the sulphurlevel is low. The conditions required for carrying out hydrogenation ofaromatic compounds are generally similar to those required forhydrodesulphurisation. However, as the reaction is an equilibrium thatis not favoured by use of high temperatures, the conditions required fordehydrosulphurisation of cyclic and polycyclic organic sulphur compoundsin a conventional plant do not favour hydrogenation of aromaticcompounds. Moreover as the design of conventional hydrodesulphurisationplants results in high partial pressures of H₂ S at the downstream endof the plant the catalyst activity is correspondingly reduced and theconditions do not lead to significant reduction in the aromatic contentof the feedstock being treated. Hence in an article entitled "Panelgives hydrotreating guides", Hydrocarbon Processing, March 1989, pages113 to 116, it is stated at page 114:

"It is a fundamental kinetic fact that at pressures for normal middledistillate desulfurizers (500 to 800 psig) it is difficult to obtainappreciable aromatic saturation. Thus, if the feedstock is far above the20% aromatics level, there is not much you can do with typicalhydrotreaters, with any catalysts that we have knowledge of, tosignificantly reduce aromatics.

You are then left with the unpalatable alternatives of higher pressureunits, aromatic extraction, and all the other alternatives."

Removal of H₂ S from a hydrodesulphurisation plant with a gas recyclesystem is normally effected by scrubbing the recycle gas with an amine.As the scrubber section has to be sufficiently large to cope with thehighest levels of sulphurous impurities likely to be present in thefeedstocks to be treated, the scrubber equipment has to be designed withan appropriate capacity, even though the plant will often be operatedwith low sulphur feedstocks. The capital cost of such scrubber equipmentis significant.

It would be desirable to provide a more efficient process for effectinghydrodesulphurisation of liquid hydrocarbon feedstocks, in particularone in which the danger of hydrocracking reactions occurring issubstantially obviated. It would further be desirable to provide ahydrodesulphurisation process in which the activity of the catalyst iscontrolled throughout the reactor in such a way that improved levels ofhydrodesulphurisation can be achieved at a given operating pressure thancan be achieved in a conventional process. It would also be desirable toprovide a hydrodesulphurisation process which permits operation in sucha way as to achieve a simultaneous significant reduction in thearomatics content of the feedstock being treated, particularly thosefeedstocks in which the aromatics content exceeds about 20%.

The invention accordingly seeks to provide a process in whichhydrodesulphurisation can be conducted more efficiently than in aconventional hydrodesulphurisation process. It also seeks to provide ahydrodesulphurisation process in which the activity of the catalyst iscontrolled favourably throughout the reactor to enable improved levelsof hydrodesulphurisation of the feedstock to be achieved. It furtherseeks to provide a hydrodesulphurisation process which enables also asignificant reduction in the aromatics content of the feedstock to beeffected simultaneously with hydrodesulphurisation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a two stage hydrodesulphurisation plantdesigned to operate using the process of the present invention.

FIG. 2 is a vertical section through a tray of the column reactor of thehydrodesulphurisation plant of FIG. 1.

FIG. 3 is a vertical section through a tray of a modified design of acolumn reactor.

FIG. 4 is a flow diagram of an apparatus according to this invention.

FIG. 5 is a diagram of a reaction tray of the apparatus illustrated inFIG. 4.

FIG. 6 is a diagram showing the relationship between the aromaticscontent of the product and temperature in an aromatics hydrogenationreaction.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to the present invention there is provided ahydrodesulphurisation process for continuously effectinghydrodesulphurisation of a liquid sulphur-containing hydrocarbonfeedstock which comprises:

(a) providing a hydrodesulphurisation zone maintained underhydrodesulphurisation conditions and comprising a column reactor havinga plurality of reaction trays therein mounted one above another, eachtray defining a respective reaction stage adapted to hold apredetermined liquid volume and a charge of a particulate sulphidedsolid hydrodesulphurisation catalyst suspended therein, and eachreaction tray having a floor at least a part of which slopes at an angleequal to or greater than the angle of repose of the catalyst particlesunder the liquid, said column reactor further comprising liquiddowncomer means associated with each reaction tray adapted to allowliquid to pass down the column reactor from that tray but to retainsolid catalyst thereon, and gas upcomer means associated with eachreaction tray adapted to allow gas to enter that tray from below and toagitate the mixture of liquid and catalyst on that tray;

(b) supplying liquid sulphur-containing hydrocarbon feedstock to theuppermost one of said plurality of reaction trays;

(c) supplying hydrogen-containing gas below the lowermost one of saidplurality of reaction trays;

(d) allowing liquid to pass downward through the column reactor fromtray to tray;

(e) allowing hydrogen-containing gas to pass upward through the columnreactor from tray to tray;

(f) recovering from the uppermost one of said plurality of reactiontrays an off-gas containing H₂ S produced by hydrodesulphurisation; and

(g) recovering from the lowermost one of said plurality of reactiontrays a liquid hydrocarbon product of reduced sulphur content.

Normally the gas entering the uppermost tray contains sufficient H₂ Sand/or the liquid feedstock contains sufficient sulphur-containingmaterial selected from H₂ S and active sulphur-containing materials tomaintain the catalyst charge thereon in sulphided form.

By the term active sulphur-containing materials there is meant materialswhich very rapidly form H₂ S under hydrodesulphurisation conditions inthe presence of a hydrodesulphurisation catalyst. Examples of suchmaterials include, for example, CS₂, COS, alkyl mercaptans, dialkylsulphides, and dialkyl disulphides.

The solid sulphided catalyst used in the process of the presentinvention is preferably selected from molybdenum disulphide, tungstensulphide, cobalt sulphide, nickel/tungsten sulphide, cobalt/tungstensulphide, sulphided nickel-molybdate catalysts (NiMoS_(x)), a sulphidedCoO-MoO₃ /gamma-Al₂ O₃ catalyst, and mixtures thereof.

Typical hydrodesulphurisation conditions include use of a pressure inthe range of from about 20 bar to about 150 bar and of a temperature inthe range of from about 240° C. to about 400° C. Preferred conditionsinclude use of a pressure of from about 25 bar to about 100 bar and of atemperature of from about 250° C. to about 370° C.

The liquid sulphur-containing hydrocarbon feedstock may comprise amixture of saturated hydrocarbons, such as n-paraffins, iso-paraffins,and naphthenes, in varying proportions. It may further comprise one ormore aromatic hydrocarbons in amounts of, for example, from about 1volume % up to about 30 volume % or more. If the feedstock has a lowcontent of aromatic hydrocarbons, then hydrodesulphurisation will be thepredominant reaction occurring. However, if the feedstock has anappreciable content of aromatic hydrocarbons, then at least somehydrogenation of these to partially or wholly saturated hydrocarbons mayalso occur concurrently with hydrodesulphurisation. In this case thehydrogen consumption will be correspondingly increased. The extent ofsuch hydrogenation of aromatic hydrocarbons will be influenced by thechoice of reaction conditions and so the degree of dearomatisation ofthe feedstock that is achieved can be affected by the reactionconditions selected.

In the process of the invention the stoichiometric hydrogen demand maythus be a function not only of the sulphur content of the feedstock butalso of the aromatics content thereof. The actual hydrogen consumptionwill be a function of the severity of the reaction conditions chosen,that is to say the operating temperature and pressure chosen. Thus, forexample, by conditions of high severity there is meant use of a highoperating pressure, a high operating temperature, or a combination ofboth. By and large the higher the temperature is to which thehydrocarbon feedstock is subjected during hydrodesulphurisation at agiven partial pressure of hydrogen, the closer will be the extent ofaromatics hydrogenation (or dearomatisation) to that corresponding tothe theoretical equilibrium concentration achievable. Thus the amount ofhydrogen consumed by the process of the invention does not depend solelyupon the nature of the feedstock but also upon the severity of thereaction conditions used.

If the feedstock is, for example, a diesel fuel feedstock then thereaction conditions used in the process of the invention will typicallybe chosen to reduce the residual sulphur content to about 0.5 wt % S orless, e.g. about 0.3 wt % S or less, even down to about 0.05 wt % S orless and to reduce the aromatics content to about 27 volume % or lower,e.g. to about 20 volume % or less. If the desired product is a"technical grade" white oil, then the process conditions will beselected with a view to reducing the sulphur content to very low levelsand the aromatics content as far as possible. Typically the aim will beto reduce the aromatics content sufficiently to provide a white oilwhich is a colourless, essentially non aromatic, mixture of paraffin andnaphthenic oils which conform to the following specification:

    ______________________________________                                        Saybolt colour    +20                                                         UV Absorbance limits                                                          Maximum absorbance                                                            per centimetre                                                                280-289 mμ     4.0                                                         290-299 mμ     3.3                                                         300-329 mμ     2.3                                                         330-350 mμ     0.8                                                         ______________________________________                                    

If the desired end product is a medicinal grade white oil complying withthe current requirements of the U.S. Department of Food and DrugAdministration, then the aim is to produce a product with a maximum uvabsorption per centimeter at 260-350 nm of 0.1, measured on adimethylsulphoxide extract using the procedure laid down in the U.S.Pharmacopoeia. Other specifications require a sample to give at most aweak colouring in a hot acid test using sulphuric acid and to give noreaction in the sodium plumbite test. To meet these stringentrequirements effectively all aromatic hydrocarbons present in thefeedstock must be hydrogenated.

In the process of the invention there will be used in amount of hydrogenwhich is equivalent to at least the stoichiometric amount of hydrogenneeded to desulphurise the feedstock and to achieve the desired degreeof dearomatisation. Normally it will be preferred to use at least about1.05 times such stoichiometric amount of hydrogen. In addition allowancehas to be made for hydrogen dissolved in the recovered treatedfeedstock.

In the process of the invention the rate of supply ofhydrogen-containing gas typically corresponds to an H₂ :feedstock molarfeed ratio of from about 2:1 to about 20:1; preferably this ratio isfrom about 3:1 to about 7:1.

The hydrogen-containing gas may be obtained in known manner, for exampleby steam reforming or partial oxidation of a hydrocarbon feedstock, suchas natural gas, followed by conventional steps such as the water gasshift reaction, CO₂ removal, and pressure swing adsorption.

Different hydrodesulphurisation conditions may be used on differentreaction trays. Thus, for example, the temperature on the uppermostreaction tray, which forms a first hydrodesulphurisation zone, may belower than on the next lower tray, which in turn may be lower than thetemperature on the next lower tray, and so on.

It is also envisaged that the temperature may be increased from tray totray from the uppermost tray to an intermediate lower tray, but then thetemperature is reduced from tray to tray on the succeeding lower trays.Thus it is possible to operate the process so that the temperatureincreases tray by tray from the uppermost tray to the intermediate tray,but then decreases from tray to tray as the liquid passes down throughthe column reactor. Under this regime, the feedstock will encounterprogressively hotter conditions under essentially the same pressure, andprogressively lower H₂ S partial pressures in passing down throughsuccessive reaction trays. Since the H₂ S partial pressure is lower onthe second tray and on the lower trays than on the uppermost tray, thecatalyst is effectively less sulphided and hence more active on thelower trays than on the uppermost tray. In this way the efficiency ofhydrodesulphurisation is enhanced, since the conditions on the lowertrays are more favourable for reaction of the remainingsulphur-containing compounds, which will tend to be the least reactivecompounds, such as polycyclic sulphur-containing compounds. In addition,by reducing the temperature on the lower trays and also enhancing thecatalyst activity on these trays, due to the lower H₂ S partial pressureon these trays, the conditions are rendered mare favourable foreffecting hydrogenation of aromatic components of the feedstock, areaction which, although promoted by an increase in hydrogen partialpressure, is equilibrium limited at high temperatures.

As the hydrogen-containing gas flowing to the uppermost tray comes fromthe next lower tray it will normally contain a proportion of H₂ S. Sincethe make-up gas is supplied below the lowermost tray, the concentrationof H₂ S in the gas is at its highest in the gas leaving the uppermosttray. The level of organic sulphur-containing compounds is lowest in theliquid on the lowermost tray, but these compounds are the leastreactive. Whilst a sufficient H₂ S partial pressure should be maintainedon the lowermost tray in order to keep the catalyst on that tray in asufficiently sulphided form to obviate the danger of hydrocracking onthat tray, the catalyst activity will tend to be highest on this tray sothat the conditions on this tray are favourable not only for effectinghydrodesulphurisation but also for effecting hydrogenation of aromaticcompounds. Hence, under suitable operating conditions, a significantreduction of the aromatic hydrocarbon content of the feedstock can beeffected, while at the same time achieving efficient removal of the lessreadily removed sulphur-containing materials, such as cyclic andpolycyclic organic sulphur compounds.

It is also envisaged that different catalysts can be used on differenttrays in the process of the invention. In this case a catalyst favouringhydrodesulphurisation, rather than hydrogenation of aromatic compounds,can be used an the uppermost tray or on the uppermost few trays, whilsta catalyst that has greater activity for hydrogenation of aromaticcompounds is used on the lower trays.

Means may be provided for withdrawing a mixture of catalyst and liquidfrom one or more trays. By providing suitable valves and pumps the samewithdrawal means can also be used to charge fresh catalyst to each tray,either in order to vary the quantity of catalyst in response to changesin feedstock or operating conditions or in order to replenish thecatalyst charge.

It is preferred that the sulphur contents of the gas and liquid feeds tothe uppermost tray are monitored to ensure that there is sufficient H₂ Spresent to maintain the catalyst in sulphided form. More often than notthe feedstock will contain sufficient active sulphur-containing materialor the hydrogen-containing gas fed thereto will contain sufficient H₂ S,or both, to maintain the catalyst in sufficiently sulphided form.However if, for any reason, there should be a dangerously low level ofH₂ S or active sulphur-containing material on the uppermost tray, then asufficient additional amount of H₂ S or of an active sulphur compound,such as CS₂, COS, an alkyl mercaptan, a dialkyl sulphide, or a dialkyldisulphide, is added to the feedstock supplied to the uppermost tray torestore a safe level of sulphur on that tray.

Normally it will suffice to provide on the uppermost tray a sulphurconcentration, in the form of H₂ S or of an active sulphur material, offrom about 1 ppm, and preferably at least about 5 ppm up to about 1000ppm. Typically the sulphur concentration may range from about 10 ppmupwards, e.g. from about 40 ppm up to about 100 ppm.

It is further preferred to monitor the sulphur concentration on at leastone lower tray, for example the lowermost tray, and possibly on eachlower tray below the uppermost tray, and to bleed into the feed to thattray, if necessary, sufficient H₂ S or sufficient additional activesulphur-containing material, such as CS₂, COS, or an alkyl mercaptan, adialkyl sulphide, or a dialkyl disulphide, to maintain the sulphurconcentration within the range of from about 1 ppm to about 1000 ppm,for example from about 5 ppm to about 100 ppm.

The liquid hydrocarbon feedstock may be, for example, selected fromnaphthas, kerosenes, middle distillates, vacuum gas oils, lube oilbrightstocks, diesel fuels, atmospheric gas oils, light cycle oils,light fuel oils, and the like.

In the reactor used in the process of the invention the reaction trayseach include a floor at least a part of which slopes at an angle equalto or greater than the angle of repose of the catalyst particles underthe liquid. Thus the reaction trays may each have a floor offrusto-conical shape whose slope is equal to or greater than the angleof repose of the catalyst particles under the liquid present on thetrays. Alternatively the reaction trays may each have a floor and asurrounding wall, the floor including an inner floor portion whichslopes downwardly and inwardly from a central portion and an outer floorportion which slopes downwardly and inwardly from the surrounding walland the slope of the inner and outer floor portions each being equal toor greater than the angle of repose of the catalyst particles under theliquid on the tray.

The gas upcomer means associated with each reaction tray may compriseone or more bubble caps of conventional design. In a particularlypreferred arrangement each such bubble cap is associated with arelatively tall riser tube, the height of which is sufficient to preventthe tray from draining the liquid in the case of temporary interruptionof gas upflow for any reason. This relatively tall riser tube is coveredby a corresponding inverted, relatively tall bubble cap. A non-returnvalve may also be fitted in such a riser tube.

If in the course of a single passage through the column reactor thedesired degree of desulphurisation or dearomatisation is not achieved,then the treated material can be re-treated in a subsequent columnreactor or other form of hydrodesulphurisation reactor, possibly undermore severe conditions.

In order that the invention may be clearly understood and readilycarried into effect a preferred process in accordance with the inventionwill now be described, by way of example only, with reference to theaccompanying diagrammatic drawings.

It will be appreciated by those skilled in the art that, as FIG. 1 isdiagrammatic, further items of equipment such as heaters, coolers,temperature sensors, temperature controllers, pressure sensors, pressurerelief valves, control valves, level controllers, and the like, wouldadditionally be required in a commercial plant. The provision of suchancillary items of equipment forms no part of the present invention andwould be in accordance with conventional chemical engineering practice.

Referring to FIG. 1 a hydrodesulphurisation plant includes a columnreactor vessel 1 provided with a plurality of reaction trays 2 eachholding a charge of a particulate sulphided hydrodesulphurisationcatalyst and a predetermined volume of liquid. A hydrogen-containing gasis admitted to the reactor vessel 1 in line 3. Spargers 4 mounted ineach tray 2 permit upward flow of gas through the liquid on each tray 2.A sulphur-containing hydrocarbon feedstock to be treated is supplied tothe plant in line 5 and is admixed with recycled liquid in line 6 toform a mixed feed in line 7 which feeds the topmost tray 2 of reactorvessel 1. Downcomers 8 allow liquid to pass downwardly through columnreactor vessel 1 from one tray 2 to the next lower tray and finally tocollect in the sump 9 of column reactor 1. Further details of one of thetrays 2 showing two different arrangements of the sparger 4 and thedowncomer 8 are described below in relation to FIGS. 2 and 3.

It will thus be seen that gas and liquid flow in countercurrent incolumn reactor vessel 1.

Each tray 2 is provided with a corresponding heat exchanger coil 10. Atemperature controller 11 is provided for each tray 2 and controls theaction of a corresponding control valve 12, which controls the flow tothe respective heat exchange coil 10 of a heat exchange medium suppliedin line 13 to an inlet manifold 14. (For the sake of simplicity only onetemperature controller 11 is shown in FIG. 1 for the topmost tray 2; inpractice each other tray 2 has its own corresponding temperaturecontroller 11). Reference numeral 15 indicates the return flow manifold,and reference numeral 16 the return flow line for the heat exchangemedium. This arrangement enables the temperature on each tray 2 to beindividually controlled.

Off-gas is recovered from the top of column reactor vessel 1 in line 17.This contains vapours of the liquid feedstock, gaseous products of thehydrodesulphurisation reaction, including H₂ S, and inert gasescontained in the feed gas in line 3. Vaporous materials are condensed inpassage through condenser 18 and a two phase mixture of gas andcondensate flows on in line 19 to a gas/liquid separator 20 providedwith a droplet de-entrainer 21.

Condensate is withdrawn from separator 20 by way of lines 22 and 23 bypump 24 and is recirculated to separator 20 in lines 25 and 26 throughflow constrictor 27 which thus creates a pressure in lines 25 and 26that is greater than that in separator 20. Part of the liquid flowing inline 25 is returned to column reactor 1 by way of lines 28, 29 and 6under the control of valve 30 which is in turn controlled by levelcontroller 31 fitted to separator 20. A selected flow of liquid isdiverted from line 28 via line 32 and through valve 33 and flow sensor34 to a selected lower tray 2; in FIG. 1 it is the bottom tray 2 towhich line 32 leads, but it could be a higher tray 2 than the bottomone. Flow through valve 33 is controlled by a flow controller 35 whichis connected to flow sensor 34. This liquid supply via line 32 to alower part of column reactor 1 provides a route by which reactivesulphur compounds can be supplied to the lower trays 2, thereby enablingthe activity of the catalyst on the lower tray 2 to be regulated, aswill be further described below.

Gas from separator 20 is purged from the plant in line 36 and passesthrough pressure let-down valve 37 to lines 38 and 39 by means of whichit exits the plant. This off-gas contains H₂ S produced as a result ofthe hydrodesulphurisation treatment and can be subjected to furthertreatment (e.g. H₂ S removal and subsequent conversion to elementalsulphur by partial oxidation).

The liquid accumulating in sump 9 is recovered in line 40 and passesthrough flow control valve 41 to cooler 42. Flow controller 43 controlsvalve 41. The cooled liquid passes, together with desorbed hydrogen(which is less soluble in cold liquid hydrocarbons than in hot liquidhydrocarbons), by way of line 44 to gas/liquid separator 45. This isfitted with a droplet de-entrainer 46 and a gas return line 47 whichleads back to the bottom of column reactor 1. The liquid collecting inseparator 45 flows in line 48 through valve 49, which is under thecontrol of level controller 50, to gas-liquid separator 51. This has adroplet de-entrainer 52 and a gas purge line 53 as well as a productrecovery line 54.

Reference numeral 55 indicates a line by means of which a controlledamount of H₂ S or of an active sulphur-containing material, such as CS₂,COS, an alkyl mercaptan of formula RSE, an alkyl sulphide of formulaRSR, or a dialkyl disulphide of formula RS-SR, in which R is an alkylgroup such as n-butyl, can be supplied, conveniently in the form of asolution in a hydrocarbon solvent, as necessary to thehydrodesulphurisation plant as will be described further below.

In operation of the plant of FIG. 1 the liquid feedstock supplied inline 5 passes through the reactor 1 and finally exits the plant in line54. In passage through the reactor 1 the organic sulphur compounds arelargely converted to H₂ S some of which exits the plant in line 54dissolved in the liquid product. Separation of H₂ S from the liquidproduct can be effected in known manner, e.g. by stripping in adownstream processing unit (not shown). Although the make-uphydrogen-containing gas in line 3 is essentially sulphur-free, theliquid flowing onto the bottom tray 2 of reactor 1 of reactor 1 willnormally contain sufficient H₂ S to ensure that thehydrodesulphurisation catalyst thereon remains adequately sulphided andso any risk of hydrocracking reactions occurring on the bottom tray 2 ofreactor 1 is minimised. On the higher trays 2 the gas feed comes from alower tray 2 and so will contain H₂ S from contact with the liquid phasein that tray. Hence there will normally be a sufficient H₂ S partialpressure at each tray 2 of reactor 1 to ensure that its catalyst chargeis adequately sulphided. If, however, for any reason the H₂ S partialpressure on any tray 2 of reactor 1 should fall below a safe level, thena suitable amount of a sulphur-containing material, preferably H₂ S,CS₂, COS, or an active organic sulphur-containing material such as analkyl mercaptan (e.g. n-butyl mercaptan), a dialkyl sulphide (such asdi-n-butyl sulphide), or a dialkyl disulphide (e./g. di-n-butyldisulphide), is supplied, conveniently as a solution in a hydrocarbonsolvent, in line 55 in order to boost the sulphur content of the feed tothe respective tray 2. As CS₂, COS, alkyl mercaptans, dialkyl sulphidesand dialkyl disulphides are readily and rapidly converted to H₂ S, itcan be ensured that the catalyst charge on each tray 2 of reactor 1remains adequately sulphided so as to remove essentially all risk ofhydrocracking occurring in reactor 1.

FIG. 2 illustrates a design of tray 2 suitable for use in a relativelysmall scale reactor 1. In this case a frusto-conical partition ordiaphragm 70 extends within wall 71 of reactor I and closes off thecross section of reactor 1 completely except for a downcomer 72 forliquid and a gas upcomer 73. The slope of frusto-conical diaphragm 70 isequal to or greater than the angle of repose of the solid particulatehydrodesulphurisation catalyst under the liquid present on tray 2.

Gas upcomer 73 includes an axial tube 74 which is open at its upper endand which is covered by a bubble cap 75. An annular member of mesh 76prevents catalyst particles from being sucked back within bubble cap 75.A cylindrical baffle 77 surrounds bubble cap 75 symmetrically and ispositioned so as to lie beneath the liquid level 78 on tray 2, theheight of which liquid level is determined by the height of the upperend of downcomer 72. A screen 79 is fitted to the top of downcomer 72 toretain catalyst particles on tray 2. Reference numeral 80 indicates thedowncomer from the next higher tray 2 (not illustrated).

Baffle 77 promotes agitation of the liquid/catalyst suspension by theupcoming gas. The vertical extent of baffle 77 is not critical butshould generally be between one third and three quarters of the verticalheight between diaphragm 70 and liquid surface 78. It is preferred thatbaffle 77 should be placed in a symmetrical or near symmetrical verticalposition. In the zone inside baffle 77 the liquid flow is generallyupward whilst outside baffle 77 the general direction of liquid flow isdownward. Preferably the area of the zone inside baffle 77 approximatelyequals the sum of the area outside baffle 77.

An anti-suckback valve 81 is fitted to gas upcomer 73.

The temperature of the liquid and catalyst on tray 2 can be controlledby means of heat exchanger coil 82.

In operation the upcoming gas bubbles through the liquid on tray 2 andagitates the mixture of liquid and catalyst, thus maintaining thecatalyst particles in suspension. Baffle 77 assists in inducing goodcirculation of liquid on tray 2. At a suitable rate of gas upflow thebubbles of gas, indicated generally at 83 maintain the majority of thecatalyst particles (indicated at 84) in suspension. (It will beappreciated that, in order not to complicate the drawing only a fewbubbles 83 and a few catalyst particles 84 are shown).

FIG. 3 illustrates an alternative construction of tray 2 of reactor 1 ofthe plant of FIG. 1. A horizontal diaphragm or partition 100 extendswithin wall 101 of reactor 1 and closes off the cross section of reactor1 completely except for a downcomer 102 for liquid and a gas upcomer103. Partition 100 has an axial frusto-conical part 104 surrounding gasupcomer 103 and an annular sloping portion 105 adjacent wall 101. Tray 2can thus retain a volume of liquid whose surface is indicated at 106 andwhose volume is determined by the height of the overflow level ofdowncomer 102 above the partition 100. Each tray 2 also supports acharge of a solid sulphided hydrodesulphurisation catalyst whoseparticles are indicated diagrammatically at 107. Such particles 107 arekept in suspension in the liquid on tray 2 as a result of agitationcaused by the upcoming gas as will be described below. To prevent escapeof particles 107 with the liquid overflowing down downcomer 102 the topof downcomer 102 is provided with a screen 108. The slope offrusto-conical part 104 and of sloping portion 105 is equal to orgreater than the angle of repose of the solid particulate catalyst underthe liquid on tray 2.

Gas upcomer 103 conducts upcoming gas to a circular sparger 109, whichsurrounds frusto-conical part 104, by way of spider tubes 110. Suckbackof liquid down upcomer 103 is prevented by means of an anti-suckbackvalve 111.

Annular draught shrouds or baffles 112 and 113 are positioned within thebody of liquid on tray 2, one inside and one outside circular sparger109 to promote agitation of the liquid/catalyst suspension by theupcoming gas. The vertical extent of shrouds 112 and 113 is not criticalbut should generally be between one third and three quarters of thevertical height between diaphragm 100 and liquid surface 106. It ispreferred that shrouds 112 and 113 should be placed in a symmetrical ornear symmetrical vertical position. In the annular zone between shrouds112 and 113 the liquid flow is generally upward whilst inside shroud 112and outside shroud 113 the general direction of liquid flow is downward.Preferably the area of the annular zone between shrouds 112 and 113approximately equals the sum of the areas inside shroud 112 and outsideshroud 113.

Reference numeral 114 indicates a downcomer from the next tray above theone illustrated in FIG. 3. The liquid level in downcomer 114 isindicated at 115, the height H of this liquid level above liquid level116 on tray 2 being fixed by the liquid level on the tray which feedsdowncomer 114 (i.e. the tray above the illustrated tray 2) plus thepressure drop through the sparger 109 on that tray (i.e. the one abovethe illustrated tray 2) and the frictional pressure drop.

The temperature on tray 2 of FIG. 3 can be controlled by means of a heatexchanger coil 117.

In an alternative arrangement (not shown), heat exchanger 117 is omittedbut an external heat exchanger is connected to the column reactorthrough which liquid drawn from tray 2 can be pumped for temperaturecontrol purposes.

The invention is further illustrated in the following Example.

EXAMPLE

The hydrodesulphurisation of a synthetic mixture of an organosulphurcompound (dibenzothiophene) dissolved in a saturated hydrocarbon solvent(n-hexadecane) is studied in the apparatus shown in FIG. 4.

The liquid feedstock consists of a solution of dibenzothiophene (DBT) innormal hexadecane (114 grams of DBT per liter of solution) which isstored in a feed tank 201. The feed tank 201 is charged with previouslyprepared solution via line 202 and then purged with dry nitrogen vialines 202 and 203. The feed tank 201, delivery line 204, metering pump205 and transfer line 206 are located in a circulating hot air chamber(not shown) thermostated at 50° C.+/-5° C. to avoid any risk of solidformation (since the melting point of n-hexadecane is about 20° C.). Theflow of liquid to a column reactor 207 is effected by metering pump 205and checked from time to time by a burette in the hot air chamber (alsonot shown). The transfer line 206 is wound with an electrical resistanceheater 208 so that the feed liquid can be preheated before entering theupper part of column reactor 207. Column reactor 207 consists of avertical metal cylinder 2.0 meters high and 7.5 cm in internal diametercontaining eight reaction tray sections 209 to 216. (The construction ofone of trays 209 to 216 is shown in more detail in FIG. 5 and is furtherdescribed below). Hydrogen gas is supplied in line 217 to the lower partof reactor 207 (above the liquid level in its sump 218) and the desiredhydrogen flow is obtained by adjustment of a mass flow controller 219.

The whole of reactor 207 is enclosed in an electrically heated forcedflow circulating hot air bath (not shown).

Liquid proceeds downwards from the uppermost reaction tray 209 tolowermost reaction tray 216 by successively overflowing from one tray tothe next tray below and at the same time hydrogen containing gas passesupwards through the trays 216 to 209 bubbling through the liquid on eachtray. Liquid collects in sump 218 at the base of reactor 207, isdischarged from the system by line 220, and is cooled by cooler 2211through which water at approximately 45° C. is passed, under the controlof valve 222 operated by level controller 223. Samples of the liquid arecollected for analysis from line 224.

A gas phase leaves reactor 207 by line 225 and is cooled by condenser226 which is supplied with water at 40° C. and the cooled gas andcondensate pass via line 227 to drum 228. The gas phase then passesthrough line 229 and upstream pressure control valve 230 which sets theoverall system operating pressure. The gas passes on to analytical flowmeasurement equipment and a flare (all not shown). The liquid collectingin drum 228 can be returned to tray 209 by line 231 under the control ofvalve 232 operated by a level sensor 233 on drum 228 or a sample can bewithdrawn via line 234 and hand operated valve 235.

In example of a design for one of the reaction trays 209 to 216 is shownin FIG. 5. The vertical walls of the reactor column are shown by thenumerals 240. The tray is also fitted with a charge/discharge tube 241which is fitted with a block valve 242 used for the loading/unloading ofcatalyst slurry to the respective reaction tray.

The horizontal cross section of the reactor column 207 is closed by afrusto-conical diaphragm represented in vertical cross section by thenumerals 243 and 244. The sloping part of the diaphragm 243 makes anangle of 30° to the horizontal. The horizontal part of the diaphragm 244is 4 cm in diameter and is pierced by vertical tubes of 4 mm internaldiameter at two locations.

Tube 245 passes through the diaphragm on the vertical axis of thereactor and projects upward into bubble cap 246 for a distance of 22 cm.The projection of tube 245 below the lower surface of horizontal portion244 is small and can be zero. Bubble cap 246 is 20 Mm in externaldiameter; a 2 mm gap between the lower edge of bubble cap 246 and theupper surface of horizontal portion 244 is closed by screen material 247consisting of fine stainless steel mesh to prevent catalyst particlesentering the inner part of bubble cap 246.

Tube 248 starts at a level 9 cm above horizontal portion 244 andproceeds downward to finish in the space below portion 244. It is cappedby mesh 249 in order to prevent ingress of catalyst particles. The levelof the top of tube 248 determines the level of liquid 250 on the tray.The lower end of tube 248 is below the liquid level on the tray beneath(or in the case of tray 216 the lower end of tube 248 is below theliquid surface in the sump 218 of the reactor 207).

In operation the tray (i.e. one of the trays 209 to 216) receives liquidfrom the tray above by a tube 248 as described above (or in the case oftray 209 from the liquid feed line 206); liquid overflows throughcatalyst screen material 249 and passes down to the next tray below. Gaspasses through tube 245, the inner part of bubble cap 246, screen 247and bubbles through the liquid on the tray; the agitation created by thebubbles maintains catalyst particles 252 in turbulent suspension. Thegas phase escaping through liquid surface 250 passes through the axialtube of the tray above (or in the case of tray 209 into line 225). Inthis way the liquid phase proceeds down from tray 209, to tray 210 andso on down to tray 216 and on to sump 218 and is thereby contactedcountercurrently with the gas phase passing through tray 216 upwards totray 215 and so on upwards to tray 209 and on to line 225.

After the apparatus is purged with nitrogen and the feed tank 201 ischarged with the feed solution the vent line 229 is opened to theatmosphere by opening a bypass line around valve 230 (not shown).

With valve 242 open each tray 209 to 216 is charged with catalyst slurrythrough line 241. In this way there is charged to each of the trays 209to 216 35 cm³ of 0.1 to 0.5 mm size range catalyst in 290 cm³ ofn-hexadecane. The catalyst is sulphided CoO-MoO₃ /gamma-alumina whichhas been previously reduced in hydrogen, cooled and immersed inn-hexadecane. 45 cm³ of n-hexadecane are used to wash any catalyst inline 241 into the tray. Valve 242 is then closed. A flow of nitrogen isestablished through line 217 and allowed to pass up column 207 and tovent to atmosphere via line 229. The nitrogen flow is changed tohydrogen and the apparatus is slowly pressurised to 35 bar (absolute),the column heater being used to increase the temperature from 30° C. to325° C. over 4 hours.

As soon as the operating pressure is reached, after about 45 minutes,the hydrogen feed rate is increased to 465 liters/hr (NTP). At thispoint a solution of 10 g/liter of CS₂ in n-hexadecane is pumped intotray 216 via line 241 at 500 cm³ /hr in order to ensure that thecatalyst is sulphided. After 3 hours the supply of CS₂ solution to tray216 is stopped and the liquid feed to tray 209 is started andestablished at 587 cm³ /hr. During this time the liquid level in sump218 increases and liquid then leaves the apparatus under the control oflevel sensor 223 and control valve 222.

The product liquid in line 224 is sampled from time to time and analysedfor dibenzothiophene by gas liquid chromatography. After twelve hours ofsteady operation the analysis shows that 99.1% of the dibenzothiophenehas been converted to sulphur free products and only non-quantifiabletraces of the tetrahydro- and hexahydro-derivatives of dibenzothiophenecan be seen on the chromatographic recording. During the next three daysoperation the dibenzothiophene conversion progressively increases to99.5% and stabilises.

COMPARATIVE EXAMPLE

A comparative experiment in which 280 cm³ of the same catalyst in theform of 1 mm diameter extrudates 1 to 3 mm long is packed into a 25 mminternal diameter reactor to give a bed of catalyst 57 cm deep. Whenthis conventional reactor is operated in the cocurrent gas and liquiddownflow mode, using the same feed composition, and with the sametemperature, pressure, and gas and liquid flow rates as are used in theExample, only 96.4% conversion of the dibenzothiophene is achieved, thusindicating a considerably poorer performance compared with thecountercurrent column operation (more than seven times the amount ofdibenzothiophene remaining unconverted).

The hydrogenation of aromatic compounds in the presence of ahydrodesulphurisation catalyst depends upon a number of factors,including thermodynamic and kinetic factors as well as the catalystactivity and its effectiveness.

From the point of view of thermodynamics the hydrogenation of anaromatic compound, e.g. an aromatic hydrocarbon, is an exothermicprocess. Moreover the extent to which the reaction will occur underparticular conditions is limited by considerations such as theequilibrium at those conditions. In general the equilibrium is lessfavourable at high temperatures. Hence it is beneficial to operate atlower reaction temperatures, if possible.

The kinetics of the hydrogenation of aromatic hydrogenation reactionsare favoured by use of high temperatures. Thus the rate of aromaticshydrogenation is increased strongly with increasing temperature, at aparticular fixed hydrogen partial pressure, provided that theconcentration of aromatics in the reaction mixture is above theequilibrium limit at the temperature concerned.

The capability of a given mass of catalyst of defined particle sizerange to perform aromatics hydrogenation is a function of the irrigationintensity applied to the catalyst particles, of the degree of sulphidingof the catalyst, and of the rates of mass transfer of H₂ and H₂ S to andaway from the catalyst surface. Generally speaking, the best propensityfor aromatics hydrogenation will be exhibited by a catalyst with a lowdegree of sulphidation which is exposed to a turbulent two phase(gas/liquid) mixed flow.

FIG. 6 is a graph indicating diagrammatically the effect of thesevarious factors upon an aromatics hydrogenation reaction. In FIG. 6there is plotted percentage aromatics in the product versus temperaturefor a given hydrogen partial pressure. Line A--A' in FIG. 6 indicatesthe variation with temperature, at a fixed hydrogen partial pressure, ofthe kinetically limited aromatics content of the product obtained from agiven feedstock with a particular aromatics content using a fixedquantity of catalyst. Line B--B' represents the equilibrium limitedaromatics content in the product from the same reaction system as afunction of temperature. At any given temperature the line XY (or X'Y')represents the excess aromatics content of the product and henceprovides a measure of the driving force required by the catalyst. Thepoint O represents the lowest aromatics content obtainable from thegiven system and is obtainable only by selecting a combination of themost favourable kinetics and the less favourable equilibrium as thetemperature increases.

If the activity of the catalyst can be enhanced in some way, e.g. bycontrolling the degree of sulphiding thereof, then a new curve, such asC--C', can be obtained, with a new lower optimum aromatics level (pointO') obtainable.

In practice crude oil derived feedstocks contain many different aromaticcompounds and sulphur compounds, each with their own hydrogenation andhydrodesulphurisation kinetics. The prior removal of the less refractorymaterials, and the removal of the associated H₂ S from the sulphurcompounds, that is possible using the teachings of the invention, makesit possible to achieve significant advantages using the process of theinvention compared with conventional hydrodesulphurisation practices.

We claim:
 1. A hydrodesulphurisation process for continuously effectinghydrodesulphurisation of a liquid sulphur-containing hydrocarbonfeedstock which comprises:(a) providing a hydrodesulphurisation zonemaintained under hydrodesulphurisation conditions including use of apressure in the range of from about 20 bar to about 150 bar and of atemperature in the range of from about 240° to about 400° C., saidhydrodesulfurization zone said zone comprising a column reactor having aplurality of reaction trays therein mounted one above another, each traydefining a respective reaction stage adapted to hold a predeterminedliquid volume and a charge of a particulate sulphided solidhydrodesulphurisation catalyst suspended therein, and each reaction trayhaving a floor at least a part of which slopes at an angle equal to orgreater than the angle of repose of the catalyst particles under theliquid, said column reactor further comprising liquid downcomer meansassociated with each reaction tray adapted to allow liquid to pass downthe column reactor from that tray but to retain solid catalyst thereon,and gas upcomer means associated with each reaction tray adapted toallow gas to enter that tray from below and to agitate the mixture ofliquid and catalyst on that tray, thereby maintaining the particulatecatalyst in suspension in the liquid on the tray; (b) supplying liquidsulphur-containing hydrocarbon feedstock to the uppermost one of saidplurality of reaction trays; (c) supplying hydrogen-containing gas belowthe lowermost one of said plurality of reaction trays; (d) allowingliquid to pass downward through the column reactor from tray to tray;(e) allowing hydrogen-containing gas to pass upward through the columnreactor from tray to tray; (f) recovering from the uppermost one of saidplurality of reaction trays an off-gas containing H₂ S produced byhydrodesulphurisation; and (g) recovering from the lowermost one of saidplurality of reaction trays a liquid hydrocarbon product of reducedsulphur content.
 2. A process according to claim 1, in which the solidsulphided catalyst used is selected from molybdenum disulphide, tungstensulphide, cobalt sulphide, sulphided nickel-molybdate catalysts(NiMoS_(x)), a sulphided CoO-MoO₃ /gamma-Al₂ O₃ catalyst, and mixturesthereof.
 3. A process according to claim 1, in which the reaction trayseach have a floor of frusto-conical shape whose slope is equal to orgreater than the angle of repose of the catalyst particles under theliquid present on the tray.
 4. A process according to claim 1, in whichthe reaction trays each have a floor and a surrounding wall, the floorincluding an inner floor portion which slopes downwardly and outwardlyfrom a central portion and an outer floor portion which slopesdownwardly and inwardly from the surrounding wall and the slope of theinner and outer floor portions each being equal to or greater than theangle of repose of the catalyst particles under the liquid present onthe tray.
 5. A process according to claim 1, in which each reaction trayis provided with a heat exchanger for controlling the temperature of themixture of liquid and catalyst on the reaction tray.
 6. A processaccording to claim 1, in which the temperature of the mixture of liquidand catalyst is increased from tray to tray from the uppermost tray toan intermediate lower tray and is decreased from tray to tray on thetrays succeeding said intermediate lower tray.